The invention relates generally to Magnetic Resonance Imaging (MRI) or Spectroscopy systems and more specifically to a radio frequency (RF) coil assembly for use in such systems, the coil assembly being invariant in its performance with respect to the angle and orientation of a main magnetic field of these systems.
Magnetic Resonance Imaging (MRI) is a non-invasive imaging technique, in which the imaged subject is kept in a static main magnetic field, known as the B0 field and the nuclei of the imaged subject are excited by the radio-frequency (RF) magnetic field known as the B1 field. Magnetic Resonance Spectroscopy is a similar technique used to identify chemical makeup of a subject. For certain MRI and Spectroscopy systems, a plurality of radio frequency coils are needed to transmit the RF energy to the nuclear magnetic moments as well as to receive the extremely small signal that comes back from the subject. The signals, referred to as magnetic resonance signals, result from reorientation of certain gyromagnetic materials of the subject, whose molecules spin or precess at characteristic frequencies. The radio frequency coils are commonly employed to image whole body, head and limb imaging in medical applications. Such techniques are also used outside of the medical imaging field, such as in part inspection, baggage inspection, and so forth.
Various coil geometries and coil arrangements are used to enhance the ability to transmit to or receive signals from the imaged or analyzed subject. Quadrature arrangements used in MRI generate two B1 fields at right angles to each other with a 90° phase shift between the two. This generates a transmitter field with a defined direction of rotation. Provided this rotation corresponds with the sense of the spin's precession, the coil excites the spins with twice the efficiency of a non-quadrature (linear) coil. During a receive phase, two independent phase-shifted signals are obtained which, following additional electronic phase shifting, can be combined to produce a signal with an improved signal-to-noise ratio (SNR). A quadrature coil is typically constructed using two independent coils wound at right angles to each other.
One variation in the quadrature arrangement is a birdcage coil. The birdcage coil is a single structure which can be driven independently at two positions 90° apart. The birdcage coil behaves like a tuned transmission line with one complete cycle of standing wave around the circumference. One advantage of this arrangement is that it is simple to produce an exceedingly uniform B1 radio frequency field over most of the coil's volume, providing images with a high degree of uniformity. A second advantage is that nodes with zero voltage occur 90° away from the driven part of the coil, thus facilitating the introduction of a second signal in quadrature which produces a circularly polarized radio frequency field. Birdcage coils usually employ between 8 and 16 elements, aligned along the direction of the static B0 field, connected between annular rings. Capacitors are placed either in each strut or between each strut in the end rings, thus forming low-pass or high pass birdcage coils.
One common limitation which exists while using these coils is that the B1 sensitive axis of the coil is generally kept in an orientation perpendicular to B0 magnetic field for maximum sensitivity and SNR. In quadrature arrangements, another limitation is that the rotation axis of the coil must match the polarization direction of the B0 field. This limitation becomes even more acute where the coil is incorporated into devices that can be moved within the magnet, for example in a catheter which uses a solenoid coil. If the axis of the solenoid coil is parallel to B0, it will detect MR signals poorly. A plurality of coils are conventionally used to overcome this limitation to achieve an insensitive orientation, but as a consequence each coil has a different spatial sensitivity. Thus the spatial location within the device having optimal signal intensity will vary as the device orientation changes.
It would therefore be desirable to have a coil structure which is symmetrical and sensitive in three orthogonal planes, and is invariant with respect to the angle and orientation of a main magnetic field of the MR system.